A further example of how genetic instability contributes to cancer progression is recent data indicating that mutation in any one of four human mismatch repair gene homologs, MSH2. MLH1, PMS1 and PMS2, predisposes to hereditary nonpolyposis colon cancer (HNPCC). We have cloned cDNA and/or genomic sequences for three mouse MutL homologs, PMS2, MLH1 and PMS1. We have derived by gene knockouts in embryonic stem cells, mice that are heterozygous and homozygous for a PMS2 null mutation. A tumor from a homozygous animal exhibits new alleles for a number of randomly chosen microsatellite loci. Genome-wide instability of microsatellite sequences is one key feature of "mismatch repair defective" sporadic tumors and tumors from HNPCC individuals. We have observed male- specific sterility in mice homozygous for PMS2 null mutation. Our overall objective is to study further the effect of "null" mutations in MutL homologs of DNA mismatch repair genes. We will concentrate on homologs of the bacterial MutL gene, PMS2 and MLH1. A primary goal is to derive an animal model for a common form of human cancer, HNPCC. We will monitor tumor formation in PMS2 heterozygous and homozygous animals. We will measure microsatellite instability in these tumors. We wilt test for cooperativity between a null allele in PMS2 and different alleles of the mouse APC (adenomatous polyposis coli) gene. We will study the effect of PMS2 mutation in the male germ line. We wilt measure the frequency of mutation in CA/GT dinucleotide repeats in sperm from normal, heterozygous and homozygous mice. In these mice we will study heteroduplex DNA repair during male meiotic recombination. To examine the basis for the male- specific seen in mice homozygous for mutation in PMS2, we will evaluate spermatogenesis primarily by cytological methods, including an assessment of meiotic progression. We will measure mutation rates in cell lines established from mouse embryos that are wild-type, heterozygous or homozygous for the PMS2 null mutation. Finally, we will derive mice carrying mutations in a second mismatch repair gene, MLH1, and compare the phenotype of these mice to those with PMS2 mutation.